Detachable index arm unit for measuring instruments



Nov. 24, 1953 H. L. BOWDITCH DETACHABLE INDEX ARM UNIT FOR MEASURING INSTRUMENTS Filed Sept. 2, 1952 2 Sheets-Sheet l .w m m m N D 5 J N T [B A Lm B n F Nov. 24, 1953 BOWDITCH 2,660,147

DETACHABLE INDEX ARM UNIT FOR MEASURING INSTRUMENTS Filed Sept. 2, 1952 2 Sheets-Sheet 2 2 33 L |9 I I I I I l yo FIGI INVENTOR. HOEL L. BOWDITCH ATTORNEYS Patented Nov. 24, 1953 UNITED STATES PATENT OFFICE DETACHABLE INDEX ARM UNIT FOR MEASURING INSTRUMENTS Application September 2, 1952, Serial No. 307,545

1 Claim.

This invention relates to measuring instruments wherein an index arm unit is driven in relation to changes in a variable condition. In particular, this invention relates to a new and improved index arm unit, for example, a pointer, a set arm, or a pen arm, for such instruments.

In industrial instruments especially, it is desirable to use lightweight and delicate driving and driven elements in order to accurately measure small power responses to changes in variable conditions.

In some instruments it is desirable to reduce the frontal face size of the instruments in order to more efficiently use multi-instrument panel space. One method of so reducing face size is to give the instrument substantial depth, and this arrangement may necessitate the use of long index arms, driven from the rear of the instrument. Such arms must be even more lightweight and delicate than previous, shorter arms in order to be as small a burden on the instrument as possible.

A problem in instrument structures is that index arms need to be easy to assemble in the manufacture of such instruments, and the arms may need to be repaired or replaced, and it is important for index arm structures and arrangements to be capable of ready assembly or disassembly. The assembly of the index arms must be sufficiently strongly held together to insure definiteness and duplication of alignment and of operative dimension when disassembled and replaced or reassembled.

In order to provide ready assembly and disassembly of the index arms, some form of snap action structure is preferable. However, since the assembly must be strongly held together, a substantial force is necessary to disengage or engage the snap action assembly. This problem is aggravated in instruments of the type previously mentioned by the fact that extra long index arms need to be extra lightweight and delicate. The application of substantial force to such arms in the act of assembling or disassembling them with respect to the instrument is very liable to materially deform or otherwise damage the index arms. v

Such extra long index arms are usually made of thin strip stock, and even though they may be reinforced by longitudinal ridges or like formations, they are still delicate in relation to the force necessary to assemble or disassemble them with respect to the instrument.

This invention provides index arm structures with snap action assembly arrangements to which 4 substantial force may be applied to accomplish assembly and disassembly thereof. This is accomplished by providing index arm structures which may be snapped in or out of assembly with the instrument by applying force transversely of the index arm, that is, in the direction of the greatest strength of the arm.

It is therefore an object of this invention to provide new and improved measuring instrument index arm arrangements for snap action assembly and disassembly with respect to strongly held assemblies wherein the index arms are unusually long, lightweight, and delicate.

Another object is to provide a snap action connection arrangement for a measuring instrument index arm in which the snap action connection is used primarily to locate and hold the assembled parts strongly together, and other means is used to provide substantial support for the index arm.

These and other objects of this invention will be in part pointed out and will be in part obvious from the text and claim here presented, and from the accompanying drawings, in which:

Figure I is a fragmentary perspective showing of a measuring instrument incorporating an index arm unit embodying this invention;

Figure II is a side elevation of the index arm unit of Figure I;

Figure III is an enlargement of a portion of the structure of Figure II, as indicated in Figure II by the numeral III;

Figure IV is a plan view of one separable portion of the index arm unit of Figures I, II, and

III; and

Figure V is a plan view of another separable portion of the index arm unit of Figures I, II, and III.

Referring to Figure I, an index arm unit I0 is shown in association with a m asuring instrument portion H. The position of the index arm unit I 0 indicates the value of a factor that varies as an incident of the operation of the instrument, for example, valve position in a flow control arrangement. The instrument portion H is contained in a housing i2 with a strip chart l3 mounted therein on rollers I4 and I5 to be driven in association with a recording pen I 6 for recording the measurement action of the instrument. This instrument is shown simply for the purpose of showing a typical application of the index arm unit It. The details of the instrument are therefore not shown. Any of the usual strip chart recording structures may be used for this purpose. However, the index arm unit It is shown broken to indicate substantial length, greater than usual in measuring instrument index arms. This extra long index arm unit is an indication that one application of the index arm unit of this invention is its use in an instrument where the frontal face size is reduced by giving the instrument substantial depth. Desirable instrument design in such acase may dictate that the index arm unitsibe driven from the rear: of the instrument, thus requiring extra long, extra lightweight index arm units.

The instrument portion 1 l is shown with a double index scale block 11 mounted, above the stri chart I3, and a control set point indicator -i 8 is arranged in associationawith one oftheindex scales on the block 11. has a free end pointer portion l9 associated with the other index scale on the, block l1.

The index arm unit 10 has a driven base orstub portion which is secured to a rotatable. shaft 2 l. The shaft 2| is driven by any suitable means, ,not, shown, in response-to changes in a: variable condition; The. driven shaft 2 l is shown in a vertical; position with the index arm unit lll noyable in ,BrghOFiZDl-ltfil plane thereby. Theindex arm :stub 20 is generally in the form of a'flat-strip with cneend bentin a U formation with the shaft 2| extending vertically, through both legs of the. U fprma tion. As a means of clamping the stub 20 l toythe shaft 21, the u formation is slotted as at through the base-of the U to connect withboth of. the shaft receiving openings in the 'U forma tion. ;.A clarnp bolt 23 is provided with a head 24 bearing on one edge of the. U formation. The bolt 23 extendsbetween the,legs of the U formagtion andbetween the shaft 2| and the base of the -U. :As may be seen in Figure IV, thebolt 23 is threaded into acover plate" 25-on the i pposing =edze-of the, U formation. Thus as the bolt 23 is tightened the slot 23 is narrowed and, the stub 20 is'clamped to the shaft 2 I.

:zStill referring to Figure IV, the stubylO-isprowided with a pair of connector-legs 28. Theselegs extend generally in the'direction of the longitudinal axis of the index arm unit in straddling relationwith respect to that axis. :Theselegsfiurther, terminate in end portions arranged transversely =ofxtheindex arm unit; and laterally spaced from the main body of the unit. The connector legs are formed with a, degree of resiliency for small movements toward and away from eachother. Duplicate connector capstans 21 are mountedon the outer ends of the connector-legs 26. As shown in Figure III, the capstans 21 are each formed with a central, reduced diameter, cylindrical portion' 28 and with frusto-conical ends 29 in tapering. enlargement from the cylindrical portion '28.

Referring again to the stub 25, and to;Figures 'I,1'I, and IV, an adjustment bolt 30 is provided. This bolt is mounted in vertical threaded relation i with the stub 20, and extends through both sides o! the U formation of the stub-2D. The bolt39 is located on the longitudinal axis of, the index arm unit, and in the stub 20 at a point adjacent the junction of the connect0rlegs26. Thisadjust- -ment bolt has a spool-like member 3| secured to,

the underside of the head of thebolt, and concentrlc therewith, which is used as a factor in the assembly of the index arm unit Ill, aswillbe explained later herein.

Referring to Figures I and V, an index arm 32 is provided for separable assembly with the. stub 20,.with this assembly forming the complete index -arm unit 10. The index arm 32 has an elongated, ilightweight flat strip as a main body, with the pointer formation; 13 at the outer end ,pf this The; index; arm. ,unit .I 0-

body, and an inner, butt end 33 as the driven and assembly end of the main body. A rigid, fiat strip connector crosspiece 34 is secured, flat to flat, to the main body of the index arm 32 at a point on that arm adjacent its butt end portion 33. The midportion of the crosspiece is fixed to the index arm 32 by apair of rivets 35. ,The configuration pf, the crosspiece is generally in the ,form of a C,

facing the butt end of the index arm. Each end of the crosspiece 34 is provided with a curved portion 36 for receiving the capstans 21 of the stub 1 2ll;in assembled relation. The curved portions 36 1 arm 32. The radii of the cylinder portions 36 are somewhatgreater than the radii of the central cylindrical portions 28 of the capstans 21. Fur

; ther, the'thickness of the crosspiece 34 is somewhat less than the length of the central cylindri- .cal' portion 26 ofthe capstans 21.. Lastly, the dis- .tance 'betweenthe centers of the curvedportions 36 is established as somewhatless than the distance between the centersof the capstans 21 in the unrestricted, unassembled condition of the stub 20 and its connector legs 26.

The above structural I details. simply mean, that the index arm 32 and the stub 26 are-detachably connectable in a snap actionassembly, through the assembly of the index armcrosspiece with the stub capstans 21, by means of the containment of the capstans: in the crosspiece cylindrical portions 28. When the index arm and stubare so assembled, the resilient stub connector legs 28 are under constrictiontoward each other because the spacing of the crosspiece portions 28 is somewhat less than the unrestricted spacing of the capstans 21. Assembly and disassembly isaccom- ,plished by applying manual force transversely of the index arm, that is, laterally oftheindex arm as awhole. To assemble; one of the capstans 21 is placed in its associated crosspiece-cylindrical portion, and the othercapstan is snapped into place in the other crosspiece cylindrical portion by forcing the crosspieceabout the first capstan as a-pivot and constrictingr-the stub connector vlegs 26 by camming pressureonthe seccndrcap- .stan by the crosspiece. strongly holds the index arm and stub together This arrangement I by the axisline 31. ;The crosspiece connections to the capstans 21 provide this effective pivotarrangement because the crosspiece thickness-is somewhat less thansthe length of the capstan'cylinder 28 andbecausethe crosspiece cylindrical configurations 35 have'radii Somewhat greater than the radii of the capstan-cylinder 28. 'Thus the index arm is drivenin ahorizontal-plane at right angles to the drive shaft 2!, andin addition, theindex arm is pivotable in:a vertical plane.

In the assembly of the index arm 32 andthe stub 20, the index arm butt end 33 is located between the ends of the adjustment spool 3 I, and is in upwardly biased engagement with the top end of the spool. This upward bias is provided by ,thevpull gravity on the elongated index arm portion on the opposite side of the index arm pivot axis 31. Thevindex arm pointer Hlmay thus be liftedagainst gravity within the limits provided by the spool ends, while its downward movement is limited by the positive stop action of the adjustment bolt 30. The index arm is flexi'ble and will bend down somewhat because of this factor, even though it is in engagement with a positive stop at its butt end. The spool ends may be separated by whatever distance is preferred for free pivotal movement of the index arm.

In the arrangement shown, the index arm may be pivoted in a vertical plane to move the indicator point I9 upwards by adjusting the bolt 30 downwards. This action may be used to lift the point [9 or simply to reduce the effect of gravity on the index arm. In some instances it may be desirable to add to the gravital effect by adjusting the bolt 30 upwards. This adjustment will result in the index arm indicator point moving downward until the index arm engages some part of the instrument. Further upward adjustment of the bolt 3!] would then result in increased pressure in this index arm engagement. With an index arm in the form of a pen arm, this arrangement would be found useful to vary the pen pressure on a recording surface. In other arrangements, for example where the index arm pivot axis 31 is vertical, the adjustment bolt 30 may still be used for positioning or variable pressure application of a pen or other index arm arrangement. Similarly, the arrangement is effective where the pivot axis 31 is horizontal and the index arm general disposition is vertical.

For the purposes of this invention, that is, the provision of a detachable index arm, the adjustment bolt 30 may be a fixed, headed pin, with 6 the index arm butt end 33 engaging the under side of the head of the p n.

This invention, therefore provides a novel measuring instrument index arm and index arm separable assembly.

As many embodiments may be made in the above invention, and as many changes may be made in the embodiment above described without departing from the spirit and scope of the invention as described herein and shown in the accompanying drawings, it is to be understood that all matter hereinbefore set forth or shown in the accompanying drawings is to be interpreted as illustrative only and not in a limiting sense.

I claim:

In a measuring instrument, a two part index arm unit comprising, in combination, a driven stub and an arm detachably secured thereto in a. snap fastener arrangement, said stub having a pair of oppositely and laterally extending resilient arms and a connection capstan on each of said arms, and said index arm having a crosspiece secured thereto and capstan receiving configurations on said crosspiece, with said resilient arms biased to hold said capstans in said configurations to provide said snap fastener arrangement.

HOEL L. BOWDITCH.

Name Date Beck May 2, 1933 Number 

